Conversion efficiency measuring apparatus of catalyst used for exhaust gas purification of internal combustion engine and the method of the same

ABSTRACT

A conversion efficiency of a catalyst is estimated by a correlation function concerning output signals of upper and down stream side air fuel ratio sensors. The conversion efficiency is measured by comparing the correlation function and a complementary value of the correlation function with a comparing standard value.

FIELD OF THE INVENTION

The present invention relates to an apparatus and a method fordiscriminating degradation of a catalyst having a function of removingdeleterious components included in an exhaust gas of an internalcombustion engine. More specifically, the present invention relates toan apparatus and a method for discriminating the degradation of thecatalyst in a system having air fuel ratio sensors located upstream anddownstream sides of the catalyst, which is located in an exhaust gaspassage of the internal combustion engine.

BACKGROUND OF THE INVENTION

A system having air fuel ratio sensors located at upstream anddownstream sides of the catalyst located in an exhaust gas passage ofthe internal combustion engine, for example, is disclosed in JapanesePatent Laid-Open No. 2-190756 published on Jul. 26, 1990 and entitled"Method and Apparatus for detecting a state of a catalyst". The priorart discloses that the degradation of the catalyst is discriminated by aphase difference, between two O₂ sensors located at upstream anddownstream sides of the catalyst used for an exhaust gas purification ofan internal combustion engine and judged in such a manner that thecatalyst is degraded when the phase difference reaches less than apredetermined value.

The prior art has a following drawback:

The sensor located at the downstream side of the catalyst is affected byelectrical noises, for example, ignition noise, so that this sensor cannot give accurate information of exhaust gases flowing through thesensor. Accordingly, the degradation of the catalyst can not be detectedaccurately only by means of the phase difference mentioned above.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an apparatus and amethod which are capable of discriminating accurately the degradation ofthe catalyst used for the exhaust gas purification of an internalcombustion engine.

The object of the present invention is attained by calculating acorrection function of outputs of air fuel ratio sensors provided at theupstream and downstream sides of the catalyst during an ordinary airfuel ratio feedback control period.

According to the present invention, the degradation cf the catalystconnected to an internal combustion engine can be discriminated withoutan actual air fuel ratio control being deviated from a set air fuelratio control as apparent from an explanation mentioned later.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic diagram of a system applied to thepresent invention;

FIGS. 2(a), 2(b), 2(c), 2(d) and 2(e) show a diagram for explainingoutput signals produced within an engine control unit based on outputsignals of air fuel ratio sensors provided at upstream and downstreamsides of a catalyst;

FIG. 3 shows a diagram for explaining calculation examples of acorrelation function and a complement value of the correlation functionwhich taken place in the present invention;

FIG. 4 shows a diagram of a characteristic between period of signal yshown in FIG. 3 and estimated value of catalyst conversion efficiency;

FIG. 5 shows a flow chart from step 510 to step 590 in one embodiment ofthe present invention;

FIG. 6 shows a flow chart from step 610 to step 640 in one embodiment ofthe present invention;

FIG. 7 shows a flow chart from step 710 to step 770 in one embodiment ofthe present invention; and

FIG. 8 shows a flow chart from step 810 to step 860 in one embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an air flow amount 4 supplied to the internalcombustion engine is measured by an air flow meter 5. Output signalsfrom a sensor 2 for detecting the temperature of the internal combustionengine, a sensor for detecting a degree of opening of a throttle valve,the air flow meter 5 and a sensor 3 for detecting rotation of the engineare inputted to an engine control unit 11. The engine control unit 11calculates a fuel injection amount and an ignition timing. A fuelinjection valve 12 supplies the fuel to the engine according to thecalculated fuel injection signal. An ignition device 13 outputs ignitionsignals to an ignition plug 15. The engine is cooled by cooling water 1.An idle speed control valve 14 is also controlled by a control signalfrom the engine control unit 11. A three way catalyst 10 convertsharmful materials passing through an exhaust gas pipe 7 to harmlessmaterials. An air fuel ratio sensor 8 is provided at an upstream side ofthe catalyst 10 and outputs a signal corresponding to an oxygenconcentration of the exhaust gas. The fuel amount supplied to the engineis controlled by feeding back the output signal of the air fuel ratiosensor 8 so as to bring the air fuel ratio closer to a theoretical airfuel ratio. An air fuel ratio sensor 9 is provided at a downstream sideof the catalyst 10 and detects the air fuel ratio thereat.

FIG. 2(a) shows a waveform of the air fuel ratio sensor 8 and acomparison level 1. A value of the comparison level 1 equals a meanvalue of the maximum and minimum values of the wave of the air fuelratio sensor 8. FIG. 2(b) shows a binary code signal which is generatedby comparing the output signal of the air fuel ratio sensor 8 with thecomparison level 1 FIG. 2(c) shows a variation of a feedback coefficientof the air fuel ratio according to the output signal of the air fuelratio sensor 8 shown in FIG. 2(a). FIG. 2(d) shows a waveform of the airfuel ratio sensor 9 and a comparison level 2. A value of the comparisonlevel 2 is decided based on a characteristic of the air fuel ratiosensor 8 relating to the feedback control of the air fuel ratio sensor 9relating to the feedback control of the air fuel ratio by the sensor 8.FIG. 2(e) shows a binary code signal which is generated by comparing theoutput signal of the air fuel ratio sensor 9 with the comparison level2. Symbols y and z in FIGS. 2(b) and 2(d) denote an interval in which acorrelation function is calculated. The region z containing the spacewhere a rectangular wave is not generated is made to equal to a fixedinterval of the feedback coefficient of the air fuel ratio. Thecorrelation is calculated by y and z.

Referring to FIG. 3, symbols y and z denote complement signals of y andz, respectively.

The correlation function φ(τ) is shown analogously as follows: ##EQU1##where, f₁ and f₂ :function of time, such as the input and output of acommunication system,

τ:time-delay parameter, and

T:time interval.

In the present invention, the correlation function φ_(j) is calculatedaccording to the following equation. ##EQU2## where, i:sampling numberof y, and

j:phase difference to i.

A complement function φ_(j) of the correlation function φ_(j) is shownas follows. ##EQU3##

In FIG. 3, waveforms of the correction function φ_(j) and the complementfunction φ_(j) of the correlation function φ_(j) are shown. The waveformof the correlation function φ_(j) and the complement function in FIG. 3show ones which are calculated in one cycle T concerning y and y,respectively.

The conversion efficiency CE of the catalyst is calculated by thefollowing equation for compensating an effect of an output ratio betweena high level output period and a low level output period of the upstreamside air fuel ratio sensor 8.

    CE∝φ.sub.max +φ.sub.max                     (4)

where,

φ_(max) =max(φ_(j))

The conversion efficiency CE is calculated by the equation (4) inprinciple. However, the conversion efficiency CE is calculated by eitherφ_(max) or φ_(max). When the conversion efficiency CE is calculatedbased on φ_(max) or φ_(max), φ_(max) or φ_(max) has to be compensated bya duty ratio of y or y signal, respectively.

Referring to FIG. 4, the portion enclosed by a dotted line denotes aregion in which the conversion efficiency of the catalyst can not beperformed effectively and the harmful materials contained in the exhaustgas can not be converted perfectly. When the estimated conversionefficiency of the catalyst is in the region enclosed by the dotted line,a driver of a car or automobile is announced by an indicator such as alamp.

Since the output of the air fuel ratio sensor 9 located at thedownstream side of the catalyst 10 has a time delay against the outputof the air fuel ratio sensor 8 located at the upstream side of thecatalyst 10, an appropriate phase difference has to be prepared in amemory (not shown) in the engine control unit 11 as a phase differencebetween the output signal of the upstream side air fuel ratio sensor 8and the downstream side air fuel ratio sensor 9. In the embodiment ofthe present invention, the above-mentioned phase difference is givenbetween the output signal of the upstream side air fuel, ratio sensor 8and the output signal of the downstream side air fuel ratio sensor 9using a wash coat catalyst 10, and φ_(max) shown in FIG. 3 is measured.The duty ratio of the rectangular wave shown in FIG. 2(b) isapproximately 50%, when a half period of the pulse width T of the signaly during 1 cycle thereof, in which the correlation is calculated, is T₂as shown in FIG. 3. When the period T₂ is changed to T₃ in a systemcarrying out the feedback control by the upper stream side air fuelratio sensor 8 on account of an error of the duty ratio of the signal y,the waveform of the correlation function φ_(j) has an estimated error Sshown in FIG. 3. For preventing the estimated error of the conversionefficiency of the catalyst based on an error of the duty ratio of thesignal y for calculating the correlation function, the coefficient CEshown by the equation (4) is calculated. The period of the signal y canbe compensated by a ratio of T₃ to T as another compensation method ofthe duty ratio. The map shown in FIG. 4 can be also alternated dependingon an engine state in case of the conversion efficiency of the catalystbeing judged.

Referring to FIG. 5, step 510 discriminates whether the present state issuitable for discriminating the conversion efficiency of the catalyst byelements of the engine driving condition, the catalyst temperature andproceeded time from the engine starting. Step 520 discriminates whetherthe estimating calculation CE of the conversion efficiency of thecatalyst is finished. Step 530 discriminates whether the open loopcontrol period I shown in FIG. 2C, which takes place before the feedbackcontrol of the air fuel ratio used for the calculation of the conversionefficiency of the catalyst is carried out, is finished. Step 540discriminates whether the output of the air fuel ratio sensor providedat the upstream side of the catalyst and used for calculating theconversion efficiency of the catalyst is sampled for a predeterminedperiod during the air fuel ratio feedback control. A usual air fuelratio feedback control takes place at step 550. Step 560 discriminateswhether the comparison level 1 is generated. The binary code signal y isgenerated by comparing the air fuel ratio sensor output at the upstreamside of the catalyst with the comparison level 1 at step 570. Step 580discriminates whether the comparison level 1 is generated for an integerperiod of at least one cycle of the signal y. Step 590 discriminateswhether the feedback control of the air fuel ratio used for the estimatecalculation of the conversion efficiency of the catalyst is finished.

Referring to FIG. 6, step 610 discriminates whether the open-loopcontrol period II shown in FIG. 2C is finished after the air fuel ratiofeedback control take place. The open-loop control periods I and II takeplace at step 620. A feedback control except the open-loop controlperiods I and II takes place at step 630. Step 640 selects the air fuelratio open-loop control.

Referring to FIG. 7, step 710 calculates the time delay corresponding tothe phase difference between the upstream side air fuel ratio sensor andthe downstream side air fuel ratio sensor. Step 720 discriminateswhether the sampling period of the output of the downstream side airfuel ratio sensor used for the catalyst is finished. Step 730 carriesout the sampling of the downstream side air fuel ratio sensor. Step 740discriminates whether the comparison level 2 is generated. The binarycode signal z is generated at step 750. Step 760 discriminates whether apredetermined time, for example, at least 1 cycle of the signal z isproceeded. Step 770 discriminates whether the sampling of the downstreamside air fuel ratio output is finished.

Referring to FIG. 8, step 810 resets the air fuel ratio open-loopcontrol period I and prepares for carrying out the estimated calculationof the conversion efficiency of the catalyst when the condition forcalculating the conversion efficiency thereof is formed. Step 820 resetsthe air fuel ratio open-loop control period II and prepares for carryingout the estimated calculation as well as the step 810. Step 830 resetsthe estimated calculation of the conversion efficiency of the catalyst.An ordinal air fuel ratio feedback control takes place at step 840. Atstep 850, the estimated calculation of the conversion efficiency of thecatalyst takes place. The estimated calculation of the conversionefficiency of the catalyst is finished at step 860.

Since the present invention judges the degradation of the catalyst atthe exhaust gas passage in the internal combustion engine by measuringthe similarity between the output change of the upstream side air fuelratio sensor of the catalyst, namely the phase difference, the amplitudeor the output waveform thereof, and the output change of the downstreamside air fuel ratio sensor concerning the same frequency component ofthe upstream and downstream side air fuel ratio sensors in the form ofthe maximum amplitude of the calculated correlation function andcomparing the maximum amplitude of the calculated correlation functionwith a reference value showing whether the catalyst is degraded, thepresent invention can measure accurately the degradation of the catalystwithout causing a deviation between the actual air fuel ratio and theset air fuel ratio.

What we claim is:
 1. A conversion efficiency measuring apparatus of acatalyst used for an exhaust gas purification of an internal combustionengine comprisinga catalyst used for an exhaust gas purificationprovided at an exhaust gas passage of an internal combustion engine, gascomponent sensing means provided at an upstream side and a downstreamside of said catalyst used for said exhaust gas purification, and meansfor calculating a correlation function based on an output signal fromsaid exhaust gas component sensing means and estimating the conversionefficiency of said catalyst for said exhaust gas purification based on asignal generated by comparing a calculated value of the correlationfunction with a comparison level used for estimating the conversionefficiency of said catalyst used for said exhaust gas purification.
 2. Aconversion efficiency measuring apparatus of a catalyst used for anexhaust gas purification of an internal combustion engine according toclaim 1, wherein further comprising a feedback control means of a fuelamount which is supplied to said internal combustion engine based on anoutput signal of the exhaust gas component sensing means provided at theupstream side of said catalyst used for said exhaust gas purification.3. A conversion efficiency measuring apparatus of a catalyst used for anexhaust gas purification of an internal combustion engine according toclaim 1, wherein further comprising a feedback control means of a fuelamount which is supplied to said internal combustion engine based on anoutput signal of the exhaust gas component sensing means provided at thedownstream side of said catalyst used for said exhaust gas purification.4. A conversion efficiency measuring apparatus of a catalyst used for anexhaust gas purification of an internal combustion engine according toclaim 1, wherein said means for calculating said correlation functionand estimating said conversion efficiency of said catalyst used for saidexhaust gas purification calculates said correlation function based on afirst rectangular wave signal which is generated by comparing an outputsignal from the exhaust gas component sensing means provided at saidupstream side of said catalyst used for said exhaust gas purificationwith a first comparison level and a second rectangular wave signal whichis generated by comparing another output signal from the exhaust gascomponent sensing means provided at said downstream side of saidcatalyst used for said exhaust gas purification with a second comparisonlevel.
 5. A conversion efficiency measuring apparatus of a catalyst usedfor an exhaust gas purification of an internal combustion engineaccording to claim 1, wherein said means for calculating saidcorrelation function and measuring said conversion efficiency of saidcatalyst used for said exhaust gas purification outputs a signal whichis generated by comparing a complement signal which is obtained byreversing said correlation function with a comparative level forestimating said conversion efficiency of said catalyst used for saidexhaust gas purification.
 6. A conversion efficiency measuring apparatusof a catalyst used for an exhaust gas purification of an internalcombustion engine according to claim 1, wherein said means forcalculating said correlation function and measuring said conversionefficiency of said catalyst used for said exhaust gas purificationgenerates an output signal for estimating the conversion efficiency ofsaid catalyst used for said exhaust gas purification by comparing atleast one signal among an absolute value of said correlation function, amaximum value of said correlation function or a phase having the maximumvalue of said correlation function with a comparison level forestimating said conversion efficiency of said catalyst used for saidexhaust gas purification
 7. A conversion efficiency measuring apparatusof a catalyst used for an exhaust gas purification of an internalcombustion engine according to claim 1, wherein said means forcalculating said correlation function and measuring said conversionefficiency of said catalyst used for said exhaust gas purificationcalculates said correlation function within an interval of integralmultiples of an air fuel ratio feedback control period.
 8. A conversionefficiency measuring apparatus of a catalyst used for an exhaust gaspurification of an internal combustion engine according to claim 1,wherein said means for calculating said correlation function andestimating said conversion efficiency of said catalyst used for saidexhaust gas purification has an open-loop control period of the air fuelratio before, after or before and after said correlation function beingcalculated.
 9. A conversion efficiency measuring apparatus of a catalystused for an exhaust gas purification of an internal combustion engineaccording to claim 1, wherein said means for calculating saidcorrelation function and estimating said conversion efficiency of saidcatalyst used for said exhaust gas purification comprises a calculationmeans for calculating said correlation function, a memory means formemorizing a comparison level which is used for estimating theconversion efficiency of said catalyst used for said exhaust gaspurification and a comparison means for comparing an output signal fromsaid calculation means with an output signal from said memory means. 10.A conversion efficiency measuring apparatus of a catalyst used for anexhaust gas purification of an internal combustion engine according toclaim 4, wherein said means for calculating said correlation functionand estimating said conversion efficiency of said catalyst used for saidexhaust gas purification measures said conversion efficiency of saidcatalyst used for said exhaust gas purification by comparing an outputsignal of said correlation function which is calculated based on saidfirst rectangular wave signal and said second rectangular wave signaland another output signal of a correlation function which is calculatedbased on a complement signal of said first rectangular wave signal andanother complemental signal of said second rectangular wave signal witha comparison level for estimating said conversion efficiency of saidcatalyst used for said exhaust gas purification.
 11. A conversionefficiency measuring apparatus of a catalyst used for an exhaust gaspurification of an internal combustion engine according to claim 4,wherein said means for calculating said correlation function andmeasuring said conversion efficiency of said catalyst used for saidexhaust gas purification generates an output signal for estimating theconversion efficiency of said catalyst used for said exhaust gaspurification by comparing at least one signal among an absolute value ofsaid correlation function, a maximum value of said correlation functionor a phase having the maximum value of said correlation function with acomparison level for estimating said conversion efficiency of saidcatalyst used for said exhaust gas purification.
 12. A conversionefficiency measuring apparatus of a catalyst used for an exhaust gaspurification of an internal combustion engine according to claim 4,wherein said means for calculating said correlation function andmeasuring said conversion efficiency of said catalyst used for saidexhaust gas purification calculates said correlation function within aninterval of integral multiples of an air fuel ratio feedback controlperiod.
 13. A conversion efficiency measuring apparatus of a catalystused for an exhaust gas purification of an internal combustion engineaccording to claim 4, wherein said means for calculating saidcorrelation function and measuring said conversion efficiency of saidcatalyst used for said exhaust gas purification has an open-loop controlperiod of the air fuel ratio before, after or before and after saidcorrelation function being calculated.
 14. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 4, wherein saidmeans for calculating said correlation function and measuring saidconversion efficiency of said catalyst used for said exhaust gaspurification comprises a calculation means for calculating saidcorrelation function, a memory means for memorizing the conversionefficiency estimated value of said catalyst corresponding to a period ofsaid first rectangular wave signal as a comparison level which is usedfor estimating the conversion efficiency of said catalyst and acomparison means for comparing an output signal from said calculationmeans with an output signal from said memory means.
 15. A conversionefficiency measuring apparatus of a catalyst used for an exhaust gaspurification of an internal combustion engine according to claim 4,wherein said means for calculating said correlation function andmeasuring said conversion efficiency of said catalyst used for saidexhaust gas purification comprises a calculation means for calculatingsaid correlation function, a memory means for memorizing previously apredetermined phase difference between output signals of the upstreamand downstream side exhaust gas component sensing means as a comparisonlevel which is used for estimating the conversion efficiency o saidcatalyst used for said exhaust gas purification and a comparison meansfor discriminating whether the phase difference between an output signalof said calculation means and an output signal of said memory meansreaches said predetermined phase difference.
 16. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 5, wherein saidmeans for calculating said correlation function and measuring saidconversion efficiency of said catalyst used for said exhaust gaspurification generates an output signal for estimating the conversionefficiency of said catalyst used for said exhaust gas purification bycomparing at least one signal among an absolute value of saidcorrelation function, a maximum value of said correlation function or aphase having the maximum value of said correlation function with acomparison level for estimating said conversion efficiency of saidcatalyst used for said exhaust gas purification.
 17. A conversionefficiency measuring apparatus of a catalyst used for an exhaust gaspurification of an internal combustion engine according to claim 5,wherein said means for calculating said correlation function andmeasuring said conversion efficiency of said catalyst used for saidexhaust gas purification calculates said complement signal of saidcorrelation function within an interval of integral multiples of an airfuel ratio feedback control period.
 18. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 5, wherein saidmeans for calculating said correlation function and measuring saidconversion efficiency of said catalyst used said exhaust gaspurification has an open-loop control period of the air fuel ratiobefore, after or before and after said complement signal of saidcorrelation function being calculated.
 19. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 5, wherein saidmeans for calculating said correlation function and measuring saidconversion efficiency of said catalyst used for said exhaust gaspurification comprises a calculation mean for calculating a reversedvalue of said correlation function, a memory means for memorizing theconversion efficiency estimated value of said catalyst corresponding toa period of said first rectangular wave signal as a comparison levelwhich is used for estimating the conversion efficiency of said catalystused for said exhaust gas purification and a comparison means forcomparing an output signal from said calculation means with an outputsignal from said memory means.
 20. A conversion efficiency measuringapparatus of a catalyst used for an exhaust gas purification of aninternal combustion engine according to claim 5, wherein said means forcalculating said correlation function and estimating said conversionefficiency of said catalyst used for said exhaust gas purificationcomprises a calculation means for calculating a reversed value of saidcorrelation function, a memory means for memorizing previously apredetermined phase difference between output signals of the upstreamand downstream side exhaust gas component sensing means as a comparisonlevel which is used for estimating the conversion efficiency of saidcatalyst used for said exhaust gas purification and a comparison meansfor discriminating whether the phase difference between an output signalof said calculation means and an output signal of said memory meansreaches said predetermined phase difference.
 21. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 8, wherein said openloop control period of said air fuel ratio control is a period more thantwo rotations of the engine.
 22. A conversion efficiency measuringapparatus of a catalyst used for an exhaust gas purification of aninternal combustion engine according to claim 10, wherein said means forcalculating said correlation function and measuring said conversionefficiency of said catalyst used for said exhaust gas purificationgenerates an output signal for estimating the conversion efficiency ofsaid catalyst used for said exhaust gas purification by comparing atleast one signal among an absolute value of said correlation functionand a reversed value of said correlation function, a maximum value ofsaid correlation function or a phase having the maximum value of saidcorrelation function with a comparison level for estimating saidconversion efficiency of said catalyst used for said exhaust gaspurification.
 23. A conversion efficiency measuring apparatus of acatalyst used for an internal combustion engine according to claim 10,wherein said means for calculating said correlation function andmeasuring said conversion efficiency of said catalyst used for saidexhaust gas purification calculates said correlation function and areversed value of said correlation function within an interval ofintegral multiples of an air fuel ratio feedback control period.
 24. Aconversion efficiency measuring apparatus of a catalyst used for anexhaust gas purification of an internal combustion engine according toclaim 10, wherein said means for calculating said correlation functionand measuring said conversion efficiency of said catalyst used for saidexhaust gas purification has an open-loop control period of the air fuelratio control before, after or before and after said correlationfunction and a reversed value of said correlation function beingcalculated.
 25. A conversion efficiency measuring apparatus of acatalyst used for an exhaust gas purification of an internal combustionengine according to claim 10, wherein said means for calculating saidcorrelation function and measuring said conversion efficiency of saidcatalyst used for said exhaust gas purification comprises a calculationmeans for calculating said correlation function and a reversed value ofsaid correlation function, a memory means for memorizing previously apredetermined phase difference between output signals of the upstreamand downstream side exhaust gas component sensing means as a comparisonlevel which is used for estimating the conversion efficiency of saidcatalyst used for said exhaust gas purification and a comparison meansfor discriminating whether the phase difference between an output signalof said calculation means and an output signal of said memory meansreaches said predetermined phase difference.
 26. A conversion efficiencymeasuring apparatus of a catalyst used for an exhaust gas purificationof an internal combustion engine according to claim 10, wherein saidmeans for calculating said correlation function and measuring saidconversion efficiency of said catalyst used for said exhaust gaspurification comprises a calculation means for calculating saidcorrelation function and a reversed value of said correlation function,a memory means for memorizing previously a predetermined phasedifference between output signals of the upstream and downstream sideexhaust gas component sensing means as a comparison level which is usedfor estimating the conversion efficiency of said catalyst used for saidexhaust gas purification and a comparison means for discriminatingwhether the phase difference between an output signal of saidcalculation means and an output signal of said memory means reaches saidpredetermined phase difference.
 27. A conversion efficiency measuringapparatus of a catalyst used for an exhaust gas purification of aninternal combustion engine according to claim 13, wherein said open-loopcontrol period of said air fuel ratio control is a period more than tworotations of the engine.
 28. A conversion efficiency measuring apparatusof a catalyst used for an exhaust gas purification of an internalcombustion engine according to claim 18, wherein said open-loop controlperiod of said air fuel ratio control is a period more than tworotations of the engine.
 29. A conversion efficiency measuring apparatusof a catalyst used for an exhaust gas purification of an internalcombustion engine according to claim 21, wherein said open-loop controlperiod of said air fuel ratio control is a period more than tworotations of the engine.
 30. A conversion efficiency measuring method ofa catalyst used for an exhaust gas purification of an internalcombustion engine, comprising:a step for measuring an output signal forman exhaust gas component sensing means provided at upstream anddownstream sides of said catalyst used for an exhaust gas purificationof an internal combustion engine, a step of calculating a correlationfunction based on said output signal of said exhaust gas componentsensing means, a step of retrieving a comparison level which is used forestimating said conversion efficiency of said catalyst of said exhaustgas purification, and a step of measuring said conversion efficiency ofsaid catalyst used for said exhaust gas purification by comparing anoutput signal of said correlation function with an output signal of saidcomparison level.
 31. A conversion efficiency measuring method of acatalyst used for an exhaust gas purification of an internal combustionengine according to claim 30, wherein said step of calculating saidcorrelation function calculates said correlation function based on afirst rectangular wave signal which is generated by comparing an outputsignal from the exhaust gas component sensing means provided at saidupstream side of said catalyst used for said exhaust gas purificationwith a first comparison level and a second rectangular wave signal whichis generated by comparing another output signal from the exhaust gascomponent sensing means provided at said downstream side of saidcatalyst used for said exhaust gas purification with a second comparisonlevel.
 32. A conversion efficiency measuring method of a catalyst usedfor an exhaust gas purification of an internal combustion engineaccording to claim 31, wherein said step of calculating said correlationfunction has a step of calculating a complement function of saidcorrelation function and said step of measuring said conversionefficiency of said catalyst used for said exhaust gas purificationoutputs a signal by comparing a signal of said complement function withan output signal of said comparison level.
 33. A conversion efficiencymeasuring method of a catalyst used for an exhaust gas purification ofan internal combustion engine according to claim 31, wherein said stepof calculating said correlation function has a step of calculating saidcorrelation function and a complement function of said correlationfunction and said step of estimating said conversion efficiency of saidcatalyst used for said gas purification outputs a signal by comparingsignals of said correlation function and said complement function ofsaid correlation function with an output signal of said comparisonlevel. .Iadd.
 34. A conversion efficiency measuring apparatus of acatalyst used for an exhaust gas purification of an internal combustionengine comprising:a catalyst used for an exhaust gas purificationprovided at an exhaust gas passage of an internal combustion engine; gascomponent sensing means provided at an upstream side and a downstreamside of said catalyst used for said exhaust gas purification; means forcalculating a correlation function based on an output signal from saidgas component sensing means for estimating the conversion efficiency ofsaid catalyst for said exhaust gas purification; and means for comparinga calculated value of the correlation function with a comparison value..Iaddend..Iadd.
 35. A conversion efficiency measuring method of acatalyst used for an exhaust gas purification of an internal combustionengine comprising the steps of:providing an output signal from a gascomponent sensing means provided at an upstream side and a downstreamside of said catalyst used for an exhaust gas purification of aninternal combustion engine; calculating a correlation function based onsaid output signal from said gas component sensing means; retrieving acomparison level for estimating the conversion efficiency of saidcatalyst of said exhaust gas purification; and comparing a calculatedvalue of the correlation function with said comparison level..Iaddend..Iadd.
 36. A conversion efficiency measuring apparatusaccording to claim 34, wherein the means for comparing includes meansfor indicating whether the catalyst is degraded or not..Iaddend..Iadd.37. A conversion efficiency measuring method according toclaim 35, further including the step of indicating whether the catalystis degraded or not based on the step of comparing the calculated valueof the correlation function with the comparison level. .Iaddend.